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Seasoning the Streets
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Seasoning the Streets

Credit: Dubravko Sori?
Source: http://www.flickr.com/photos/11939863@N08/3794105536
License: CC BY-NC 3.0

If you live in a place with cold enough winters, then you are probably familiar with the sight of trucks dumping salt onto the roads in order to melt the snow and ice. For the most part, this is the same exact salt that you might sprinkle on your eggs, although it may be mixed with sand and small amounts of other ionic compounds. Salt can also be used as a preventative measure; snow is less likely to accumulate on a sidewalk that has been salted. Is the salt somehow releasing heat and causing the ice to melt? Why is this an effective strategy? The answer lies in the fact that solutions have different physical properties than pure liquids.

News You Can Use

  • The freezing point of any liquid decreases when a solute is dissolved in it. This strategy is frequently used in cold climates to make it more difficult for water to freeze.
  • Credit: jdj150
    Source: http://www.flickr.com/photos/jdan/377655073/
    License: CC BY-NC 3.0

    The salt is dispersed behind salt trucks across the entire road. Notice that the salt is being spread to prevent snow from accumulating and freezing [Figure2]

     

  • Freezing point depression is a colligative property, which means that the change depends on the total number of dissolved particles but not their identity.
  • Table salt (sodium chloride) and other strong electrolytes are particularly useful for this purpose because each unit dissociates into cations and anions, resulting in more total dissolved particles.
  • Learn more about using salt to de-ice roads in the following video:

 http://www.youtube.com/watch?v=HlDCzasEqAc

Show What You Know

With the links below, learn more about freezing point depression. Then answer the following questions. (For all questions, assume ionic compounds dissociate completely. In other words, the van’t Hoff factor should be an integer.)

  1. If you purchase four 750 gram jars of table salt (NaCl) and dump them all onto 1,000 kg of snow (a reasonable mass for about 6 inches of fresh snow on a typical two-car driveway), how much would the freezing point be lowered after all of the salt has dissolved?
  2. What mass of NaCl would be required to decrease the freezing point of 1,000 kg of snow by 5.0°C?
  3. Sometimes CaCl2 is used along with NaCl as a de-icing compound. How much is the freezing point of one kilogram of water lowered per gram of dissolved CaCl2? Per gram of dissolved NaCl?
  4. Water can be used as a coolant for car engines in warm climates. However, if there is a possibility that the external temperature might go below 0°C, a coolant with a lower freezing point is needed. Mixtures of ethylene glycol (C2H6O2), water, and small amounts of other compounds are commonly used instead. What would be the freezing point of a mixture in which 0.50 kg of ethylene glycol is mixed with 1.0 kg of water?

Image Attributions

  1. [1]^ Credit: Dubravko Sori?; Source: http://www.flickr.com/photos/11939863@N08/3794105536; License: CC BY-NC 3.0
  2. [2]^ Credit: jdj150; Source: http://www.flickr.com/photos/jdan/377655073/; License: CC BY-NC 3.0

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